The Sovereignty of God is the biblical teaching that all things are under God's rule and control, and that nothing happens without His direction or permission. God works not just some things but all things according to the counsel of His own will (see Eph. 1:11). His purposes are all-inclusive and never thwarted (see Isa. 46:11); nothing takes Him by surprise. The sovereignty of God is not merely that God has the power and right to govern all things, but that He does so, always and without exception. In other words, God is not merely sovereign de jure (in principle), but sovereign de facto (in practice).
"What do we mean by [the sovereignty of God]? We mean the supremacy of God, the kingship of God, the god-hood of God. To say that God is Sovereign is to declare that God is God. To say that God is Sovereign is to declare that He is the Most High, doing according to His will in the army of Heaven, and among the inhabitants of the earth, so that none can stay His hand or say unto Him what doest Thou? (Dan. 4:35). To say that God is Sovereign is to declare that He is the Almighty, the Possessor of all power in Heaven and earth, so that none can defeat His counsels, thwart His purpose, or resist His will (Psa. 115:3). To say that God is Sovereign is to declare that He is "The Governor among the nations" (Psa. 22:28), setting up kingdoms, overthrowing empires, and determining the course of dynasties as pleaseth Him best. To say that God is Sovereign is to declare that He is the "Only Potentate, the King of kings, and Lord of lords" (1 Tim. 6:15). Such is the God of the Bible." A. W. Pink, The Sovereignty of God, chapter 1.
God's sovereignty in salvation
God's sovereignty in salvation means that He saves whom He will, and those whom He saves owe nothing to anything in or of themselves. They are saved because God graciously chose them in eternity and regenerated and called them in history. They cannot even take credit for their faith because it is the gift that He Himself sovereignly bestows.